The present disclosure relates to a sheet feeding device for feeding sheets, and to an image forming apparatus incorporating a sheet feeding device.
Image forming apparatuses such as multifunction peripherals, copiers, printers, and facsimile machines incorporate a sheet feeding device for feeding placed sheets. The sheet feeding device includes a regulating plate. The regulating plate is slidable to abut opposite edges of sheets in the direction perpendicular to the sheet transport direction. The position of the regulating plate corresponds to sheet width. Thus, with a variable resistor whose knob is interlocked with the regulating plate, the width of placed sheets can be detected. Inconveniently, through repeated sliding, the movable contact and the resistive element inside the variable resistor rub against each other and wear. Thus, as the sheet feeding device continues being used, the variable resistor changes its characteristics, possibly leading to erroneous detection of sheet width.
To cope with the changing characteristics of the variable resistor resulting from the wear of the contact and the resistive element inside it, the following technique is known. In a printing device, the width of sheets fed by a sheet feeder is detected by a variable resistor interlocked with a side fence, and the width of sheets transported through a transport passage is detected by a CIS line sensor; the result of detection by the variable resistor and the result of detection by the CIS line sensor are compared together to determine the sheet width.
For the feeding of sheets to print on, a sheet feeding device such as a manual feed tray or a sheet cassette is incorporated in image forming apparatuses. Image forming apparatuses that has a scanning capability, such as multifunction peripherals, copiers, and facsimile machines, are often provided with a document transporting device for feeding a placed document to the reading position. A document transporting device is considered a kind of sheet feeding device.
A sheet feeding device includes a pair of cursors (also called “regulating plates”, “guide members”, or “side fences”) for aligning, and regulating the position of, placed sheets. The cursors are slidable, and can be moved to abut opposite edges of sheets. This mechanism permits the stack of sheets to be held in a regulated position and the placed sheets to be kept in a proper position. It also helps prevent sheet jams (stuck sheets) and prevent skew in printed or read images.
The cursors are slid to the position that suits the size of placed documents. In a fashion interlocked with the sliding of cursors, a contact inside a variable resistor is moved (the knob position is changed), and thereby the resistance value of the variable resistor is changed. The output voltage value of the variable resistor reflects the cursor position (sheet width). Thus, based on the magnitude of the output voltage value of the variable resistor, the width (size) of placed sheets can be detected. On the other hand, through repeated movement of the cursors, the contact and the resistive element inside the variable resistor keeps wearing; thus the variable resistor changes its characteristics across the ages. Thus, inconveniently, the longer the sheet feeding device is used, the more likely the width of placed sheets is to be detected erroneously.
According to the known technique mentioned above, sheet width is detected not only by a variable resistor but also by a line sensor (CIS image sensor) provided in a transport passage. That is, erroneous detection by the variable resistor is corrected with the result of detection by the line sensor. Unfortunately, this known technique is impractical for the following reasons: (1) it is not applicable to apparatuses having no line sensor; (2) rarely is a line sensor provided in a transport passage in practice; (3) additionally providing a line sensor and a circuit for processing the output of the line sensor increases the production cost of an image forming apparatus; and (4) the need for an expensive line sensor with a resolution high enough to properly correct the result of sheet width detection by a variable resistor further increases the production cost of an image forming apparatus.